1. Technical Field
Embodiments of the subject matter disclosed herein generally relate to systems and methods for using underground seismic sensors for collecting seismic data and, more particularly, to mechanisms and techniques for ghost reduction in seismic acquisition.
2. Discussion of the Background
Land seismic data acquisition and processing may be used to generate a profile (image) of the geophysical structure under the ground (subsurface). While this profile does not provide an accurate location for oil and gas reservoirs, it suggests, to those trained in the field, the presence or absence of such reservoirs. Thus, providing a high-resolution image of the subsurface is important, for example, to those who need to determine where oil and gas reservoirs are located.
Traditionally, a land seismic survey is performed in the following way. Seismic sensors (e.g., geophones, hydrophones, accelerometers, etc. or a combination of them) are electrically connected to each other and then deployed on the ground or below the ground. After all the seismic sensors have been deployed, one or more seismic sources are brought into the field and actuated to generate the seismic waves. The seismic waves propagate through the ground until they are reflected and/or refracted by various reflectors in the subsurface. The reflected and/or refracted waves propagate to the seismic sensors, where they are recorded. The recorded seismic waves may be used, among other things, for seismic monitoring of producing oil fields.
Time-lapse (or 4D) seismic monitoring of producing oil fields is an accepted method for optimization of field development and product recovery, providing significant improvements in recovery rates and savings in drilling costs. Time-lapse seismic reservoir monitoring is the comparison of 3D seismic surveys at two or more points in time. Time-lapse seismic reservoir monitoring also has potential for increasing the ability to image fluid movement between wells.
A traditional configuration for achieving a 4D seismic monitoring is illustrated in FIG. 1. FIG. 1 shows a system 10 for the acquisition of seismic data. The system 10 includes receivers 12 positioned over an area 12a of a subsurface to be explored and buried at the same depth below the surface 14 of the Earth. A number of vibroseismic sources 16 are also placed on the surface 14 in an area 16a, in a vicinity of the area 12a of the receivers 12. A recording device 18 is connected to the receivers 12 and placed, for example, in a station-truck 20. Each source 16 may be composed of a variable number of vibrators, typically between 1 and 5, and may include a local controller 22. Alternatively, the source may be a shallow buried explosive charge or other known devices for generating a seismic source, e.g., a metal plate placed on the ground and hammered with a hammer. A central controller 24 may be present to coordinate the shooting times of the sources 16. A GPS system 26 may be used to time-correlate the sources 16 and the receivers 12.
With this configuration, sources 16 are controlled to generate seismic waves, and the plurality of receivers 12 record waves reflected by the oil and/or gas reservoirs and other structures. The seismic survey may be repeated at various time intervals, e.g., months or years apart, to determine changes in the reservoirs. Although repeatability of source and receiver locations is generally easier to achieve onshore, the variations caused by changes in near-surface can be significantly larger than reservoir fluid displacement, making time-lapse 4D seismic acquisition and repeatability challenging. Thus, variations in seismic velocity in the near-surface are a factor that impacts repeatability of 4D surveys.
Several onshore time-lapse seismic case studies have shown the advantage of buried acquisition when looking at weak 4D signals (see Meunier et al, 2001, “Reservoir monitoring using permanent sources and vertical receiver antennae: The Céré-la-Ronde case study,” The Leading Edge, 20, 622-629, or Forgues et al, 2010, “Benefits of hydrophones for land seismic monitoring,” 72nd Conference and Exhibition, EAGE, Extended Abstracts, B034, the content of both of which are incorporated herein by reference). Although the seismic repeatability is improved when sources and sensors are buried, a part of the wave field (the up-going part) is still transmitted through the weathering layer and reflected at the surface. These surface reflected waves, often called “ghosts,” are affected by the near surface variations and can vary in time. In the case of daily seismic monitoring, small reservoir variations that are desired to be measured can be spoiled by the near surface waves that fluctuate in time due to temperature and moisture variation, because the waves coming from the reservoir interfere with the near-surface waves. In marine acquisition, several strategies have been developed for deghosting data using the streamer configuration.
However, the presence of the ghosts in the recorded seismic data remains a problem for the existing acquisition methods. Further, there is a need to improve the 4D seismic repeatability, increase the frequency content of the seismic data and reduce the number of sensors. Thus, there is a need for a system and method that address the above noted deficiencies of the current art.